High fat diet and enduring obesity pose a more dangerous threat in lupus patients than in general population. Obese lupus patients show much higher inflammation markers and risk of heart attack. The link between high fat western diet and accelerated atherosclerosis is clinically well established. Recent studies also show evidence that healthy eating patterns enhance immune function and reduce inflammation in obese patients. The inability of a large population to eat healthy signifies the need to explore medical interventions for diet-induced cardiovascular diseases in lupus patients. However the molecular and cellular mechanism of how high fat diet dysregulates the immune system is still unclear and thus hindering efficient therapy. B1 B cells reside in the peritoneal cavity and omentum, in direct contact with accumulated and inflamed visceral adipose tissue. B1 B cells are the source of natural IgM antibodies that protect from atherosclerosis through masking oxidized LDL, inhibiting uptake by macrophage and preventing foam cell formation. Dietary effect on B1 B cells that may lead to accelerated atherosclerosis has never been studied. Based on our preliminary study, this proposal aims to test the hypothesis that high fat western diet and chronic obesity may over-stimulate and disarm B1 B cells. Even worse, it may transform protective B1 B cells into self-attacking B cells. The result of this study will reveal new molecular and cellular mechanisms underlying diet-induced arthrosclerosis and provide new avenues for future design of B cell-targeted immune therapy. INNOVATION: This is a novel study where B1 B cells are for the first time investigated as a link between diet-induced obesity and atherosclerosis. The novel concept that high-fat-diet-stimulated B1 B cells may alter the balance of inflammatory and regulatory T cells will be tested using a novel animal model. A newly developed assay will replace traditional flow cytometry-based analysis of B1 B cells that will revolutionize B1 B cell study. Short-term high fat western diet might activate B1 B cells to produce atheroprotective IgM, cytokines and promote Treg cell differentiation. However chronic western diet consumption and obesity would switch protective B1 cell HYPOTHESIS: functions to pro-inflammatory response leading to the aggravation of atherosclerosis. SPECIFIC AIMS: Aim #1: Determine the extent to which diet-induced obesity dysregulates B1 B cell functions~ Aim #2: Determine how B1 cells from obese gld/ApoE-/- mice may affect Treg vs. Th17 balance. IMPACT: We propose to investigate an important yet unexplored immunologic component that may be the key intermediary between diet-induced obesity and heart disease in lupus patients. The result of this study will unravel unique aspects of B1 B lymphocytes in the abdominal cavity and omental milky spots and will provide a novel mechanism, as well as a non-static view of B cell-mediated protection and pathogenesis. The knowledge obtained from this study will be fundamental for the future design of B cell subpopulation-tailored risk-assessment, prevention and therapeutic intervention at different disease stages.